(i) Field of the Invention
The present invention relates to a characteristic change of an AGC circuit in a radio receiver for receiving a broadcast wave such as FM.
(ii) Description of the Prior Art
Heretofore, a radio receiver has possessed an AGC circuit in order to adapt itself to the level change of a radio wave (an RF signal). FIG. 1 is a block diagram illustrating a constitutional example of a conventional FM radio receiver. The FM broadcast wave (the RF signal) sent from a broadcasting station is received by an antenna, and the received RF signal is then input to an antenna tuning circuit 2, in which a signal in a predetermined frequency range is selected. The output of the antenna tuning circuit 2 is input to an RF amplifier 3 and then amplified therein. In general, this RF modifier 3 comprises MOSFET. Next, this amplified RF signal is input to an RF tuning circuit 4, in which a signal having a predetermined frequency is selected. A tuning frequency in this RF tuning circuit is changable, and so the tuning frequency is changed to select a desired station. The output of the RF tuning circuit 4 is input to a mixer 5, and in this mixer 5, it is mixed with a signal of a local oscillation frequency from a local oscillator 6 to become a signal (an IF signal) having an intermediate frequency. It is to be noted that the signal from the local oscillator 6 depends upon the signal selected in the RF tuning circuit 4, and the frequency of the output from the mixer 5 is always constant (10.7 MHz) irrespective of the frequency of the desired station to be selected.
The thus obtained IF signal is input to an IF amplifier 8 via a filter 7 which allows the only signal having the highly selective constant frequency (10.7 MHz) to be passed therethrough. This filter usually comprises a ceramic filter, and it can remove most signals other than the signal of the desired station. Afterward, the output of this filter is amplified by the IF amplifier 8, and a predetermined frequency is selected by a filter 9 again. Detection is then carried out by a detecting circuit 10 to obtain an audio signal.
Such a radio receiver has an AGC loop in order to prevent excessively large signals from being input to circuits constituting the radio receiver such as the amplifiers 3, 8 and the mixer 5. That is, the output signal of the mixer 5 is detected by a detecting circuit 11 to monitor an output signal level of the mixer 5. This detection may be carried out for the input signal of the mixer 5. Next, this detecting circuit 11 feeds a signal regarding the detection result to an AGC circuit 12. When the level of the received signal is high, this AGC circuit 12 controls an attenuator 13 by the signal from the detecting circuit 11 to lower the level of the signal to be input to the RF amplifier 3 and simultaneously controls the RF amplifier 3 to lower the output level of the RF amplifier 3. This attenuator 13 usually comprises a pin diode, and the value of a signal which flows to an earth side is controlled by the pin diode to lower the level of the input signal of the RF amplifier 3.
Furthermore, the RF amplifier 3 comprises a dual-gate-MOSFET, and the RF signal is input through a first gate while the control signal from the AGC circuit 12 is input through a second gate to the RF amplifier 3. When the signal level is high, a bias voltage to be applied to the second gate of the FET is lowered to decrease an amplification ratio of the FET. As is apparent from the foregoing, the level of the signal to be input to the amplifier and the mixer is lowered in accordance with the level of the received signal, whereby the proper signal level can always be achieved.
Next, the output of the detecting circuit 10 is input to an output section 14, in which a voice corresponding to the input signal is output through a speaker or the like.
However, if the level of a radio wave from a station adjacent to the desired station is very high, two-signal interference occurs in the radio receiver having the above-mentioned AGC loop. That is, if a signal having the very high radio wave level is present in the vicinity of the desired station, the signal level of the output from the mixer 5 becomes high, and the RF signal is attenuated by means of the AGC circuit 12. As a result of such an attenuation, the signal of the desired station is also attenuated, so that the level of the obtained audio signal lowers inconveniently. Thus, it has been suggested that when the output level of the detecting circuit 10 is detected and it becomes apparent that the level of the desired station is low, the sensitivity (AGC sensitivity) of the AGC circuit 12 is lowered and a start point of the attenuation is retarded. Such a constitution permits elimination of the two-signal interference.
However, radio receiver interference also includes inter modulation (IM). For example, when there are two frequencies (f1, f2), a signal having a frequency of 2f2-f1 is produced. If the two frequencies f1, f2 are present at relatively near frequencies, a signal generated by the inter modulation has about the same frequency as in another broadcasting station in a certain case, so that inter modulation interference (IM interference) occurs. The signal generated by such an IM interference is output as the signal having the frequency of the desired station, and so it appears as the output of the detecting circuit 10. Therefore, if there is a control system for lowering the AGC sensitivity in the case that the output of the detecting circuit 10 is small as described above, the signal generated by such an IM interference is amplified and output.
In particular, the IM interference can be prevented by attenuating the RF signal, and therefore, owing to the operation retard of the AGC circuit as described above, the radio receiver more easily undergoes this IM interference. In the conventional radio receiver, the AGC sensitivity is regulated on the balance of the inhibition of this two-signal interference and the inhibition of the IM interference, but it is difficult to sufficiently inhibit both types of interference.